The ontogenetic role of the gonadal steroids on morphogenetic and subcellular aspects of sexual differentiation of the perinatal reproductive axis will be further defined in long-term organotypic cultures of the rodent hypothalamus and other regions of the central nervous system. The morphological parameters of neuronal differentiation and development will be studied in living and stained preparations at both the light and electron microscopic levels and correlated with biochemical studies of the steroidal effects. Information will also be obtained by fluorescence and peroxidase immunocytochemistry, and by 3H-steroid and 3H-thymidine autoradiography. These studies are designed to further characterize the morphological aspects of the steroidal effects and to further define the developmental importance of both pre- and post-natal estrogen and androgen in regions of the central nervous system containing high-affinity steroid receptors during their "critical period" of neurogenesis. This unique model system can provide new information, heretofore unobtainable, on the organizational role and mechanisms of action of these steroids on sexual differentiation of reproductive function and on brain development in general.